[Feature] Access to request headers

## Summary

I would like to see possibility to access request headers like: `{{appsmith.request.headers}}`

## Motivation

[Authelia](https://www.authelia.com/docs/home/architecture.html) (authentication server) set headers on proxy level about user auth status (Remote-Users,Remote-Groups) which I want to read on appsmith and control user access to resources.
More details [here](https://www.authelia.com/docs/deployment/supported-proxies/#how-can-the-backend-be-aware-of-the-authenticated-users) how it "integrates" to appsmith.

## Additional Context

My need for now is related to request headers but I think that making available information like cookies from request data could be useful too.